Impedance is a complex quantity. As such, it has two components: either magnitude and phase, or real and imaginary parts (i.e., resistance and reactance). These alternative forms of a complex quantity are equivalent.
Both components of impedance of an electrochemical cell or battery are meaningful. One can gain insight into many properties such as cranking power, state-of-charge, percent capacity, temperature, and physical condition by analyzing measurements of complex impedance obtained at selected "spot" frequencies. Heretofore however, complex impedance could only be determined with bridges or expensive laboratory instruments that were poorly suited to measuring batteries in the field (e.g., E. Willihnganz and Peter Rohner, "Battery Impedance", Electrical Engineering, 78, No. 9, pp. 922-925, September, 1959; see also David Robinson, "Electrochemical Impedance Spectroscopy in Battery Development and Testing", BATTERIES INTERNATIONAL, 31, pp. 59-63, April 1997).
R. S. Robinson, in PCT International Publication WO93/22666, teaches a method for measuring complex impedance of batteries in service. However, the apparatus he discloses is an FFT-based commercial signal analyzer (HP 3562A) and the disclosed method uses existing battery currents as excitation--currents that may not contain the frequency or frequencies desired.
Devices for measuring battery impedance described in the patent literature have been generally limited to apparatus that actually determines only one quantity. For example, patent references can be found for field apparatus that purports to measure battery "impedance" (e.g., U.S. Pat. No. 4,697,134 to Burkum et al, U.S. Pat. No. 5,773,978 to Becker) and battery "resistance" (e.g., U.S. Pat. No. 3,753,094 to Furuishi, U.S. Pat. No. 3,676,770 to Sharaf et al, U.S. Pat. No. 5,047,722 to Wurst et al). However, none of these patents disclose apparatus that measures two quantities. The present patent addresses this deficiency by disclosing a practical method, and its apparatus implementation, for accurately measuring real and imaginary parts of cell/battery impedance under actual field conditions--conditions that often include a high degree of electrical noise.